1. Field of the Invention
The present invention relates to a 3-D optical sensing and imaging apparatus using an optical illumination source comprising Vertical Cavity Surface Emitting Lasers (VCSEL), and in particular, a lightfield optical source including 2-D planar array of VCSELs.
2. Related Background Art
There are many applications such as military and civilian surveillance, security and monitoring equipment, medical imaging, biometric imaging, gesture recognition input devices for interactive video games, motion activated input devices, automotive safety devices, manufacturing environment, just to name a few, that depend on accurate proximity (or distance) measurement and imaging. One common element of apparatus used in these and similar other applications is three-dimensional (3-D) imaging of objects (or human subjects). Current proximity sensing imaging and scanning apparatus requiring sophisticated and complex optical sources, detectors and complex processors limit applications to just a few areas due to cost consideration. 3-D proximity sensing, imaging and scanning will come within the realm of everyday applications as the apparatus become less complex without compromising performance and cost.
Early methods to capture 3-D images of objects used stereoscopic cameras that basically work on similar principles as human eye. More specifically, two images are recorded at slightly different angles and the images are superimposed to generate a 3-D image. The method revisited more recently is described in a United State Patent Application Publication No. 2012/0218389 by Nowak et al on Aug. 30, 2012. One major requirement in this technique is that the distance between the two cameras have to be carefully calibrated using a complex procedure which limits the application to limited environment.
A more recent technology that has emerged for distance measurement is time of flight measurement of light. An object is illuminated using a short pulse of light and the reflected light measured by a high speed detector. The arrival time of the incident light pulse to the object and the reflected light pulse from the object to the detector provides a measurement of distance from the source to the detector. Accurate distance measurement with high resolution requires a high intensity light pulse with short rise time and a high speed detector. In a recent non-patent literature publication entitled “Kilometer-range, high resolution depth imaging via 1560 nm wavelength single-photon detection” published by A. McCarthy et al. in Optic Express Vol. 21, No. 7, April 2013, pp. 8904-8915, OSA, a sophisticated time of flight measurement is described. While the image resolution is quite high, it is only possible by using a customized superconducting narrow wire single-photon detector. Clearly, such sophisticated apparatus and technique is well out of the realm of everyday applications.
Simple methods for proximity or distance detection of objects involve illuminating an object with a light source, such as an LED, and measuring the intensity of the light reflected back. The reflected light intensity reduces as the object is moved further away from the light source and detector. The method relies on good object illumination from an intense highly collimated optical source as well as known reflectance of the objects. Differences in reflectance and even shapes of objects tend to change the detected light resulting in errors in distance measurement. The sources currently available have for a reasonable cost have low intensity and/or are highly divergent which limits the distance that can be accurately measured as the reflected intensity becomes lower than the detection limit of the detector or is similar to ambient light levels.
Recent developments in high speed cameras have led to the development of three dimensional imaging systems using pulsed illumination. The region is illuminated by a short light pulse and the camera records images at different delay times. The images are then analyzed to produce a three dimensional image record. As in all time of flight systems one of the key components is the short pulse high intensity source. In addition, increasing the intensity improves the signal to noise ratio and enables objects at greater distances to be measured and imaged with higher resolution. The spectral properties of the light source are also important since spectral filtering of the light reflected to the sensor can reduce background noise from ambient light.
Most modern apparatus tend to combine different methods hybrid imaging method approach to enhance precision and reliability of images. One drawback of a hybrid method is that each method requires an optical source that is specialized for that particular method and is therefore impractical to implement. In a WIPO Patent Application Publication No. WO2013/127975 by IEE International Electronics & Engineering S.A on Sep. 13, 2013, a hybrid three-dimensional imager is described where a specialized illumination source using a VCSEL array chip to generate spatially coded structured light field is employed. It is disclosed that when a transistor for each VCSEL is integrated with the laser array chip, matrix addressing of the array is feasible, except it becomes impractical for very large arrays. The publication further discloses that a separate chip with driver circuits may alternatively bonded to the laser array by flip-chip bonding. Furthermore, structured light may be changed to a smooth flat illumination mode by positioning a switchable diffuser in front of the laser surface. Although the publication discloses use of VCSEL source for time of flight, stereovision and 2-D imaging, it does not specifically describe how different methods are combined within the framework to generate a 3-D image.
In a U.S. Pat. No. 8,320,621 issued to McEldowney on Nov. 27, 2012, a depth projector system including a light source using integrated VCSEL array to illuminate a capture area is described. A 3-D image is generated by a dispersive optical element (DOE) in the capture area. A 3-D camera senses the 3-D image. While the apparatus disclosed therein embodies 3-D imaging methods, it requires very sophisticated equipment and computing technologies to recognize and track human gestures and movements suitable for a gaming environment. The apparatus and method described therein is not particularly suited for simple applications mentioned earlier, or at a lower cost to expand usage to application other than gaming.